AzureKeyVault@2: remove duplicate %AZP25 encoding

[onetocny]

Task name: AzureKeyVault@2

Description:

This issue occurs on both release and build pipelines. In general the problem is that since certain moment build agents started to require % character coming in as values of variables be encoded as %AZP25. See more details here. Originally this was not done automatically by azure-pipelines-task-lib in tl.setVartiable. Thus it was implemented by AzureKeyVault@2 task itself (all % are encoded as %AZP25 before calling setVariable). Later this functionality was introduced even by azure-pipelines-task-lib so that latest versions of tl.setVariable does this encoding automatically. Once we have updated AKV task to use the latest version of azure-pipelines-task-lib % character started to be encoded two times. See this example:

1. Original value of variable: "hello%world",
2. Encoded by AKV: "hello%AZP25world" (% was replaced by %AZP25),
3. Encoded and stored by task lib: "hello%AZP25AZP25world" (% was again replaced by %AZP25)
4. When retrieved by tl.getVariable: "hello%AZP25world" (%AZP25 was replaced by %).

Documentation changes required: N

Added unit tests: N

Attached related issue: https://dev.azure.com/mseng/AzureDevOps/_workitems/edit/2019418

Checklist:

• Task version was bumped - please check instruction how to do it
• Checked that applied changes work as expected